1. Field of the Invention
This invention relates to a method and apparatus for developing electrostatic latent images in electrophotographic and electrostatic recording machines such as copiers and printers.
2. Prior Art
In electrophotographic and electrostatic recording machines such as copiers and printers, visible images are formed by causing a latent image-bearing body such as a photoconductor drum to bear an electrostatic latent image and feeding a developer onto the latent image-bearing body for visualizing the latent image. From the standpoints of size reduction of the development unit, cost reduction and reliability, it is advantageous to use a one-component developer in the development step. The one-component developer is often called a toner.
A variety of development methods are known in the prior art for visualizing electrostatic latent images by feeding a toner to a latent image-bearing body such as a photoconductor drum for adhering the toner to the latent image on the drum. Included are a powder cloud method of spraying toner particles, an impression developing method of feeding toner particles in direct contact with the latent image-bearing surface of a photoconductor drum, a jumping development method of charging toner particles and effecting development under the impetus of an electric field of a latent image without contact with the latent image-bearing surface of a photoconductor drum, and a magnetite development method of delivering a magnetic conductive toner in contact with the latent image-bearing surface of a photoconductor drum.
Referring to FIG. 1, the impression developing method is briefly described. A developing roller 1 having a conductive elastomer layer is placed between atoner feed roller 2 for feeding a toner 4 and a photoconductor drum 3 having an electrostatic latent image borne thereon. The developing roller 1 is in contact with or slightly spaced apart from the photoconductor drum 3. Upon rotation of the developing roller 1, photoconductor drum 3, and toner feed roller 2 in the directions shown by arrows, the toner 4 is fed from the feed roller 2 onto the surface of the developing roller 1 and regulated into a uniform thin layer by a doctor blade 5. The thin layer of toner is then delivered from the developing roller 1 to the photoconductor drum 3 to adhere to the latent image whereby the latent image is developed into a visible toner image. The toner image is finally transferred from the photoconductor drum 5 to a record medium 7, typically paper in a transfer section 6. Also included is a cleaning section 8 having a cleaning blade 9 for scraping off the toner left on the photoconductor drum 3 after the transfer step.
In reversal development as used in printers, the toner must be charged to the same polarity as the charge on the latent image-bearing drum. In general, organic photoconductors are used as the latent image-bearing drum. Since the organic photoconductors have a negative charge potential, the toner must also be charged negative. Where positive charging photoconductors such as amorphous silicon photoconductors are used, the toner must be charged positive since the charge potential of these photoconductors is positive. Since charging of the toner largely depends on triboelectric charging with the developing roller surface, the compatibility between the toner and the developing roller must be controlled by a proper design of the materials of both the toner and the developing roller. Incompatibility between the toner and the developing roller results in initial images suffering from fog and image memory, especially in a hot, high humidity environment or cold, low humidity environment. Even when the initial image is satisfactory, the incompatibility will invite problems such as lowering of image density and charge quantity as image printing is repeated.
One common approach in the prior art for controlling the compatibility between a toner and a developing roller is as the toner side control by blending a charge control agent therein in accordance with the materials of a layer-forming blade and a developing roller and as the developing roller side control by selecting a material therefor on the basis of triboelectric charging sequence.
These compatibility control techniques, however, have the following problems. With respect to the developing roller side control, the control relying solely on the triboelectric charging sequence is insufficient. In the case of conductive elastomeric rollers, conductive powder and salts are added for imparting conductivity, but can alter charging properties. Also, in the case of elastomeric rollers, a choice of material is limited in order to avoid contamination of the photoconductor. With respect to the toner side control, the additive (charge control agent) does not exert a quantitative effect and toner formulation matching with the blade and developing roller becomes a time-consuming procedure. The extent of control is limited partially because of CCA contamination.
Also in the prior art, for a proper combination of a developing roller and a toner, it is impossible to evaluate their compatibility without undue experimentation because there are available no compatibility judging methods other than actual image printing and measurement of toner's charge quantity.